Looking on google, it appears LE4 won't run on OSX 10.11 without the use of a steinberg installer tool - which is now a dead link - If you can find it then it'll come down to luck whether it works or not i fear!

Depending on what you do, REAPER is a good lightweight DAW that you could try to confirm if the issue is your machine:-http://reaper.fm/index.php
Many use it for free as an alternative, so if you're on a budget it may be worth a look and it does have basic MCU support so you can try that also.

It's my understanding that a registered copy of Cubase entitles you to run all the previous versions of that product under a single license. So even if you bought version 10, it covers you for 6,7,8,9 installs etc. But i'm not 100% sure on that, or how far it extends back if true.

Thanks. I'll give Reaper a look and see if that changes anything.
Is there any benefit to possibly building my own Windows machine (cheaper I presume) versus either getting secondhand MAC versus buying a new MAC?

Cubase runs great on windows and you’ll get more for your money in regards to raw performance, also if you’re on a budget they’re easier to upgrade over time too as you can do it bit by bit. I’m currently moving from Mac to PC for audio and it’s been brilliant, picked up an i7 Fujitsu Celsius machine from eBay for under 200 GBP to ‘test the water’ and not looked back since. It even runs old hardware that no longer runs on MacOS (thanks Apple!) which was a nice surprise.

Negatives are that as you currently use an iMac so you’d need a screen too, and depends if you have any knowledge of windows as to how easy you’ll find setup to be. You may hate windows for all I know lol

It’s sometimes cheaper and hassle free to buy a ready made system than build your own nowadays too. I’ve just had it with Apples pricing and stupid ideas to lock you out of upgrading your own machine.

I have always used Windows machines at work, but have always had MACs at home. My first computer was an Apple IIE.
While there are some things about the Windows machines that I have hated at work, they are generally specc'd to do everything I presume versus being specc'd to just be a studio workhorse.
I am not sure of all the ins and outs of building my own machine. i do know of a shop where I live and I trust this guy pretty well, so I may ask him some questions as to what he knows of and what it would cost for him to be build me something.
I will trawl around on the topics here and see if I can find out enough information to do something myself.

Well I have good news. For me anyway. Since rebuilding the machine and reloading everything, I have been able to get the MCU working correctly.
Not sure how I found it, but it works a whole lot better by not being connected in Apple MIDI Setup. I have it on the screen, but have not connected it.
In Cubase I input that the Mackie Control is connected via the ESU1808 MIDI for both input and output and the MCU works really well.
Initially I had it connected this way and connected in Apple MIDI Setup and then I used MIDI Monitor to watch what was happening. I was getting MIDI signals, but things weren't making sense. For some reason, I disconnected it in the Apple Setup app and then watched MIDI Monitor and was getting an Invalid message.
Went into Cubase and checked the connection for the MCU and it was Not Connected as expected. Told it to use the ESU1808 MIDI for input and output and everything works. Oh, except the Assignment Display readout. (and I don't know why)
Opened up the Demo project which is 39 tracks. 4 I think are VSTs and the rest are recorded tracks. There are also various effects and such.
According to iStatpro, the CPU goes to about 93% and a bit more sometimes, but everything works and sounds as it should.
So for the time being, I will be staying with the G5.

So hopped on again to look at some things as I was having a Cubase License issue for some reason. I've now got rid of that.
I think I have found why I am getting screwy control from the Mackie. Yes it seems to be intermittent.
For some reason there appears to be massive MIDI latency sometimes. It isn't all the time and I must admit that I am having trouble understanding why.
I just had a tutorial project loaded in Cubase. Press Play and away it goes. Stop the playback after awhile and the Time Display keeps on going. Why? Because I have stopped at Bar 20 for example, but the Time Display is only at 15 for example. The Time Display eventually stops once it has 'caught up' to where Cubase is up to. The Playback LED indicator also changes from Play to Stop at this time.
MIDI Monitor also confirms that MIDI Out from the MAC keeps going while the Mackie 'catches up'.
I am also getting delays with Muting a track for example. Cubase reacts at the button press, but the LED on the button can take awhile before it lights.
The issue is not the Mackie sending MIDI, but the MAC sending MIDI back to the Mackie.
I recall getting similar results when I tried running the Mackie through my MOTO microLite.
Both the ESU1808 and the microLite connect to the MAC via USB. USB 1.1 in this case.
Somehow I think I am back at having to look at a better MAC again.
That brings the next issue again of having to get a MAC that can handle Cubase 10.
So back to looking at an i7 as 'skijumptoes' suggested and then Cubase 10 I suppose, because I am not sure if you can buy anything else now?
I don't know whether to go through the exercise of putting all the software onto this laptop and then trying everything through it?
That would be moving from:
PowerPC G5 running 10.4.11 and USB 2.0
to
Intel Core 2 Duo @ 2.26GHz running 10.6.8 and USB 2.0
My brain hurts.......................

Well after some more mucking around tonight, I now have the MCU connected through the MOTU microLite and it isn't missing a beat.
Happily plays the large Cubase demo project. I don't know what was going on before, but now it seems fine.